The NDIR technique utilizing characteristic absorption bands of gases in the infrared has been widely used for decades in the gas analyzer industry for detection of these gases. Such gas analyzers utilize the principle that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term “non-dispersive” as used herein refers to the apparatus used, typically a narrow-band optical or infrared transmission filter instead of a dispersive element such as a prism or diffraction grating, for isolating for the purpose of measurement the radiation in a particular wavelength band that coincides with a strong absorption band of a gas to be measured. The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas sensors are also very sensitive, relatively stable and easy to operate and maintain. In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, short-lived and generally nonspecific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one of the best there is, NDIR gas sensors still have not enjoyed widespread high volume usage to date. There are three main reasons for this.
First, there are several applications in existence today that would require a very large number of gas sensors typically running into millions of units. One of the most outstanding examples is a personal methane (CH4) sensor worn or carried by a miner working underground in mines for detecting concentrations of gas approaching explosion limit levels when a so-called “methane pocket” is encountered and opened up during coal excavations. Since methane is odorless, there is no way for miners working underground to be warned of this deadly situation when a “methane pocket” is opened up without a personal methane sensor. If miners continue to excavate coal deposits and thus generate sparks as usual without heeding this dangerous working environment, an explosion can eventually be triggered underground when the lowest explosion limit (LEL) of methane is quickly reached.
A parallel situation might also exist when miners encounter an underground “water reservoir” while excavating (coal or otherwise). In such a situation, unless miners can by some means heed this potential flooding danger, thereby stopping work and evacuating the work site immediately, flooding could take place if miners continue to excavate and unknowingly open up a “water reservoir.” When flooding actually takes place, not only the workers working in the immediate area can face drowning, workers in neighboring sites can also be drowned. Furthermore, flooding of part or all of underground working tunnels and the subsequent necessary cleanup operations can impose a very large financial burden upon the affected mine. It is highly unlikely that the opening up of an underground “water reservoir” is very sudden and leads to immediate flooding of the work site without any early warning. Rather, it is far more likely that cracks leading to a “water reservoir” are first exposed during the excavating operation. In addition to small amounts of water seeping out which could be unnoticed, a large amount of water vapor from the underground reservoir will rush out into the atmosphere of the immediate work site. The amount of water vapor in the air at the immediate work site would rise very suddenly without any apparent reasons. If miners are equipped with a sensitive personal Dew Point (water vapor) sensor, this sudden rise in water vapor pressure in the air where they work can be detected. If the rate of water vapor pressure increase in the air is detected to exceed a certain high and unexplainable level, miners can then be immediately warned of the danger of encountering an underground “water reservoir” and they should stop work promptly, evacuating the work site and notifying the authorities above ground for an immediate investigation.
But gas sensors to be deployed in such applications, namely methane and dew point (water vapor) sensors, must be extraordinarily reliable. However, just about all gas sensors ever designed and manufactured to date, irrespective of what technology is being employed, invariably have significant output drifts over time. For this reason, sensor maintenance cost concerns render today's NDIR gas sensors unqualified to be used in such an application since they all require re-calibration maintenance service every six months to a year without exception in order for them to stay accurate over time as effective alarms.
The second reason why today's NDIR gas sensors do not enjoy widespread high volume usage has to do with their size. At present they are typically several inches in length, width and height dimensions, and such dimension are generally considered to be too big. Even if such sensors overcome their output drift reliability problem, which so far they have not, their physical dimensions remain a significant impediment to their utilization and must be drastically reduced to gain usefulness as underground warning devices in mines. Although the size of NDIR gas sensors has indeed been greatly reduced to just a couple of inches in all three dimensions in the last few years, they still are too big, and they have to be further reduced, preferably to just thumb-sized scales, in order to remove their size hindrance in the currently discussed high volume usage application.
The third and final reason why NDIR gas sensors do not enjoy widespread high volume usage is their unit production cost which has been too high for the application discussed above. About four decades ago, an NDIR gas sensor (e.g. medical CO2) was sold for more than $10,000.00 each. By the early 1990's, the unit selling price for an NDIR gas sensor (e.g. CO2) dropped to less than $500.00. Today the unit selling price of an NDIR gas sensor (e.g. CO2) goes for about $200.00, reflecting the fact that the unit production cost for such a sensor has dropped to just around $50.00 or less. But even this relatively low and reasonable unit production cost today is still too high for the above discussed application. For this application the unit production cost for an NDIR methane or dew point (water vapor) gas sensor has to be under US$10.00.
Since the three main reasons why NDIR gas sensors do not enjoy widespread high volume usages today, particularly for the application of protecting miners working underground from deadly methane explosions and/or flooding, are still not under control for elimination or remedy as conjectured in the discussion above, an object of the present invention is to eliminate these three reasons altogether. The current invention will render the outputs of NDIR gas sensors stable over time, will reduce these sensors to a very small size and finally will reduce the unit production cost of these NDIR gas sensors to just a few US dollars. The current invention is particularly well-suited for use in the application of producing methane and dew point sensors for use underground in mines for eliminating the danger of future methane explosions and/or flooding.